CGEM-DX vs. Atlas
Posted 27 October 2013 - 11:35 AM
There will be a Sky Watcher 100ED APO riding it along with a DSLR & finder scope ( I realize I won't need it with the CGEM-DX). I would like to... at some point be piggy backing or mounting seperatly a Celestron C6-C8. Although I'm fine using my old video tripod & head and adapter to manually view until I find a used GOTO mount for my viewing. Maybe I should be thinking seperate instead of doing it all or piggybacking (interested in your comments on that)...I have a Celestron C90 as a finder scope now that I use for polar alignment on my Astrotrac rig.
Reasons I like the Atlas:
-Never loose PA or goto availability once PA is achieved.
-Seems to be sturdier mount for less?
-Belt Driven mod potential?
-Grass is greener? My familiarity with Celestron may be tempting me to try other stuff undeservedly?
Reasons I like the CGEM-DX:
-I've used the Nexstar system and am famiiar with it albeit quite vaguely. As a techie I'm not really worried with software but I was able to navigate the software intuitively -only used it for two nights while at a star party but found it very easy to use.
-I do like the build quality of Celestron gear and assuming the same on the CGEM-DX?
-Seems to be a heavier mount, but is it more rigid? Says it is but what are your experiences?
There are a couple of things I don't like about either; Trivial I know, but I wish they were white to match my scope and refractor. At some point I'll be converting the old treehouse in my backyard to a make shift observatory. I'll be installing a concrete pier into the ground so I'll need the mount to detach from the legs, somthing I assumed both would do honestly.
So I know that is a lot, but I'd appreciate your feedback. Especially if your at a point where you have hit a problem with a similar setup and can advise me. I like to feel like I am making informed purchases. I ALWAYS LIKE TO BUY with a little headroom, WHILE BEING PRACTICAL as I very seldom buy things for hobbies, but when I do I want it to last and I'll usually use it until the wheels fall off.
I've been a sky watcher for about three to four years now and this will be my first GOTO mount. I realize it's a beast for the first GOTO but I've moved up and up with my camera equipment till I'm at the limit of what a homemade barn door, an Astrotrac & DSLR can do. It's time for a refractor and GOTO mount. I'm moving my DSLR astro gear which consists of several heavy duty heads and the astrotrac to move into this bit of astrophotography. My current 40d with mod (bought three years ago off craigslist)is sold and a Hap Griffin 60D is on it's way to him for his IR AP mod.
I feel like I've been doing this forever and haven't even scratched the surface. I lurked for years and kept my composure, building my own barn door tracker, and finally I got my hands on an Astrotrac. After about four months with exposing on it and managing some nice Nebula & Galaxy shots I realized I could go no further with it camera wise.
So here I am. I'm excited and glad I didn't jump into a telescope four years ago. I learned so much building my own rig and then seeing it perfected. Now I want presision. I know how to get the image I want and where to find it.
I appreciate all of your time and advise!
Posted 27 October 2013 - 01:12 PM
I started off on the CG-5 and quickly fell in love with the Nexstar controller. The synscan controller is a little bit clunkier but it can still do everything the nexstar can. But once you decide to play with EQMod you might as well sell the hand controller because you'll never use it except for maybe a quick visual session.
I believe the DX has a beefier tripod but I find the Atlas tripod more than adequate.
It carries my setup without issues, I very rarely have to throw out a sub because of mount issues. This image is a bit dated but you get the idea.
I haven't used the CGEM but there are a few people in the astro clubs I participate in using them. Some of them are happy, others have the cogging issues and seem to spend a lot of time cursing.
Posted 27 October 2013 - 02:13 PM
Posted 27 October 2013 - 03:54 PM
- Celestron hand controllers and software are excellent.
- externally controllable via wifi dongles ( read mobile device and star map).
- StarSense compatibility ( plate solving auto calibrator)
Posted 27 October 2013 - 04:22 PM
Thanks for the reply Dan! So how much would you say that telescope & finder combo weigh? How much is the most you've put on it?
I'd say around 30-35 lbs or so. Most I ever put on it was an 8" f8 but I only did visual with that.
Posted 27 October 2013 - 07:13 PM
I can also say that the C11 rides much better on the DX then it did the Atlas. The Atlas always seemed to be straining a bit when it moved the load around the sky.
Just my 0.02 cents. No charge LOL.
Posted 27 October 2013 - 09:40 PM
Posted 27 October 2013 - 09:42 PM
Posted 28 October 2013 - 03:17 PM
Posted 28 October 2013 - 05:43 PM
Posted 28 October 2013 - 06:58 PM
I really appreciate all of the comments guys. I'm going back and forth and doing more research. I must
admit that the lure of the Nexstar system is pulling me into the celestron DX camp.
And don't forget about NexRemote!! That is a great program and I use it each and every observing session in the OBS.
Good luck in your decision.
Posted 28 October 2013 - 09:59 PM
I am not sure if Celestron considers cogging and the 8/3 to be grounds for a warranty claim, and as Mark / stew57 has experienced, your replacement mount may not be any better than the older one.
For these reasons, I'd go with the Atlas.
Posted 28 October 2013 - 10:05 PM
Posted 28 October 2013 - 10:21 PM
Posted 28 October 2013 - 11:22 PM
Didn't the fix the cogging issue with a firmware update aimed at folks with the issue? That's what I've been reading. I'm a master at bricking electronics... No wait, I mean at upgrading firmware. haha.
But srsly, I did think the issue was sorted.
Posted 29 October 2013 - 12:03 AM
The graph shows the 8/3 error 75% of the PE. The PE curves repeat each 3 worm rotation because of the 8/3 error. That means the PEC can not be relied on to reduce the PE past the 8/3 error. In fact it may add to the error at certain places in the worm cycle. If my 8/3 error were not so great and abrupt the mount would guide much better.
This data was collected at a very light load (12 lbs), when the load is increased by using the C11 and guidescope the PE runs around +-20 arcseconds with the 8/3 being just a tad more than the 75% seen here but much harder to control. Celestron has stated they believe the average CGEM/DX has an 8/3 error that runs no greater than 10 arc seconds and should be less than 5. There have been user reports of 8/3 error exceeding the integer harmonic however.
These are the thoughts on the 8/3 error by Celestron in reply to a conversation with a Celestron engineer on the teamceleston site;
"First, the 8/3 gear. Probably 90% of the customers that buy a CGEM do not even know what PE is, much less how to measure it on their mount. Of those 10%, that know about PE, most of them (mistakenly) believe you can't/shouldn't use use PEC with an guide scope, or at the very least wouldn't see a benefit from doing so. For them an 8 arcsecond term that varies over 2.5 minutes is easily guidable, smaller in fact than some of the other non-periodic terms. So that leaves about 1 percent of all CGEM owners who are inconvenienced by the 8/3 gear. "
I need to add a big YMMV!
This is the PE of the first CGEM I had with an 8 lb load. With the C11 and equipment it was 180 arcseconds;
This is a 20 second shot with the C11 at 5. In all fairness this was at the worst part of the worm cycle. It could get a approx. 30 second shot with oval stars if you hit just the right spot on the worm cycle.
It took couple of tries but Celestron did replace the mount eventually and then did again when the replacement had a problem they couldn't fix. The 3rd mount is functioning quite well excepting the 8/3 error.
Posted 29 October 2013 - 12:11 AM
And, yes I knew it was "cogging" but the autocorrect on my iPad really is too smart for its own good.
Thanks again for the data!
Posted 29 October 2013 - 12:11 AM
mine was.. guiding graph not too pretty, and i wouldn't want to image at anything close to 1000mm with it. but for a 600mm or so FL it would have worked. although it's sold now.
Posted 29 October 2013 - 06:33 AM
Runningman snapped at sidewalk outreach. C11 at F10 2 min guided with minimal gain (as previous object had been orion and didn't want to change gain settings). I could have played with settings and gotten much better stars and PHD was calibrated on a different part of the sky. This was a passerby's request and time delay is not good for outreach. This pic for AP not good, for live video acceptable. That is why focus was not fussed with.
Posted 29 October 2013 - 08:01 AM
My whole reason for this mounting a AP. As my FL is 550mm with a little bit added by my crop sensor, I'm VERY concerned as to what is acceptable and what is not.
Posted 29 October 2013 - 08:01 AM
The Atlas worked great, I just didn't care for the hand control. It guided well, was relatively easy to learn the controller and is reliable.
The CGEM/Nexstar system is just better for me. I get good guiding up to 1600mm with my AT8RC (15min subs are my longest) and have had none of the issues you repeatedly read about here. The Polar Alignment routine works great for me as well and allows me to quickly set up with my limited time available this time of the year (if it actually clears here in Michigan!). In the astrphoto group in my club, 2 other CGEM's have worked just as good and one of them has done up to 5 min guided exposures at 2000mm with good results (C8). All 3 of our mounts are good, if not great, imaging mounts for what we use them for. All are 3 years old or newer.
There are good and bad stories for every mount in this price range so you just have to go with the features that are best for you. YMMV as has been said many times.
Posted 29 October 2013 - 06:30 PM
At <600mm FL and minimal weight you will get good results. I would think you will still have to guide though, but you could use a miniguider or finder turned guidescope. But then again your mount may be better. The old YMMV still applies.
Posted 29 October 2013 - 08:39 PM
Posted 29 October 2013 - 08:45 PM
8/3 is a periodic error with an irrational period (2.667X or 8/3 times faster than the fundamental) that cannot be removed by periodic error correction. This 8/3 error is due to the gearbox of the motor in both the CGEM and DX.
For "most" CGEMs according to Celestron the 8/3 is of small magnitude, under 8 arc-seconds peak to peak, but on some - like mine and Mark's - it was huge (over 25" peak-to-peak).
The cogging is the tendency of the declination motor to resist small movements (to "cog" as if it has detents) which gives problems when guiding in declination. Supposedly fixed now with new firmware, I have no way to check as I sold my CGEM.